Pump for hydraulic actuator systems



Jan. 4, 1955 E. L. BAUGH EI'AL 2,698,515

PUMP FOR HYDRAULIC ACTUATOR SYSTEMS Filed April 13, 1950 3 Sheets-Sheet 2 JNVE/YTO'QS B46357? z. aqua 054055 0. wALLncE er -muah 77/5/12 ATTORNEYS Jan. 4, 1955 E, L. BAUGH ETAL 2,698,515

PUMP FOR HYDRAULIC ACTUATOR SYSTEMS Filed April 13, 1950 3 Sheets-Sheet 3 BY L United States Patent PUMP FOR HYDRAULIC ACTUATOR SYSTEMS Everett L. Baugh and De Loss D. Wallace, Dayton, Ohio, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application April 13, 1950, Serial No. 155,692

7 Claims. (CI. 6052) This invention relates to improvements in power driven fluid pumps of the rotary type.

It is among the objects of the present invention to provide a unitary power driven fluid pump consisting of a rotary pump, an electric motor for driving the pump, and a fluid tank or reservoir, all secured together in a compact unitary structure of minimum weight and size.

A further object of the present invention is to provide a unitary device as described in the aforegoing paragraph, said device having means for controlling reversing of the pump and motor caused by back pressure in the fluid delivery line connected to the pump, immediately after the motor driven pump has stopped operating, such control substantially eliminating the undesirable belching noises resulting from sudden, uncontrolled reversal of fluid flow under back pressure and the resultant reversal in pump and motor operation.

Further objects and advantages of the present invention will be apparent from the following description, reference being bad to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

Fig. l is a plan view of the device with a portion of the electric motor broken away.

Fig. 2 is a side view of the device as shown in Fig. 1.

Fig. 3 is an end view of the device.

Fig. 4 is a diagrammatic view showing the device operatively connected to several work elements it is adapted to actuate.

Fig. 5 is a plan view of the pump housing.

Fig. 6 is a fragmentary sectional view taken along the line and in the direction of the arrows 66 of Fig. 5. This view shows the passages connecting the fluid reservoir with the intake port of the pump and the fluid flow control valve in this passage.

Fig. 7 is a sectional view taken substantially along the line and in the direction of the arrows 77 of Fig. 5.

Fig. 8 is a side view of the part shown in Fig. 5.

Fig. 9 is a view of the bottom side of the part shown in Figs. 5 and 8.

Fig. 10 is a view of the part shown in Fig. 5 and in the direction of the arrow 10 in Fig. 5.

Fig. 11 is a detail, fragmentary sectional view taken along the line and in the direction of the arrows 1111 in Fig. 10.

Fig. 12 is a view of the pump housing with the port plate applied thereto.

Fig. 13 is a detail, flat view of the port plate or disc.

Fig. 14 is a sectional view taken substantially along the line and in the direction of the arrows 1414 in Fig. 13.

Fig. 15 is a sectional view taken substantially along the line and in the direction of the arrows 1515 in Fig. 13.

Fig. 16 is a view of the pump with the cylinder plate or disc and its contained vaned rotor in position.

Fig. 17 is a view of the rotor and its substantially radial vanes.

Fig. 18 is a flat view of the cylinder disc or plate.

Figs. 19, and 21 are similar fragmentary sectional views illustrating the fluid flow control mechanism associated with the outlet passage of the pump. The Fig. 19 shows the mechanism in full shut-oil position, Fig. 20 shows the mechanism in normal pressure fluid flow establishing position and Fig. 21 showing the mechanism in excessive pressure fluid flow controlling position.

Referring to the drawings and particularly the Figs. 1,

"ice

2 and 3, the unitary mechanism is illustrated as consisting of an electric motor 20 with an extension housing 21 in which the shaft 28 of the motor is rotatably supported. The pump housing 22 is adapted to be secured to housing 21 by bolts 23. Sandwiched between the adjacent faces of housing 21 and 22 and clamped tightly therebetween by bolts 23, are two disc-shaped plates, the one designated by the numeral 24 and engaging housing 21 being the cylinder or what is sometimes called the pump barrel, while 25 designates the port plate or disc engaging the pump housing 22. The pump housing has a circular base plate 26 which forms the cover plate for the fluid tank or reservoir 27, sealingly secured to said base plate in any suitable manner. Fluid outlets for receiving pipes 30 and 31 and fluid return ports for receiving pipes 32 and 33 are provided in the pump housing. Pipes 30 and 31 direct fluid, under pressure to fluid actuated work devices 34, 35 and 36 as shown in Fig. 4.

The piston in cylinder 35 is actuated in either direction by fluid pressure while the pistons in cylinders 34 and 36 are moved in one direction by fluid pressure and in the other direction by a spring within the cylinder. The one end of cylinder 35 is connected to a selector valve 38 connected by pipe 42 with the fluid pressure supply pipe 30. Valve 38 is also connected to the fluid return pipe 32 by pipe 42a. The other end of cylinder 35 is connected to a selector valve 39 having a pipe 43 leading from it to the fluid pressure supply pipe 31 and a pipe 43a leading to the fluid return pipe 33. Valves 38 and 39 may be of a unitary structure or may be mechanically interconnected so that when one, for instance valve 38, is operated to permit fluid from pipe 30 to flow into the one end of cylinder 35, said valve closes pipe 42a. At the same time valve 39 is actuated to close pipe 43 and open pipe 43a so that the fluid in the other end of the cylinder is directed to the fluid return pipe 33 as the piston in cylinder 35 is moved by fluid pressure from pipe 42, toward said other end of the cylinder. When valve 39 is actuated to connect pipe 43 with the said other end of the cylinder 35, pipe 43a is closed and at the same time valve 38 is actuated to keep pipe 42 closed and to open pipe 42a. Normally valves 38 and 39 close both fluid pressure pipes 42 and 43 respectively.

Pistons of cylinders 34 and 36 when spring actuated, exhaust their contained fluid respectively through pipes 4130 and 4431 as permitted by control devices 37 and 40, respectively, which control the flow of fluid to and from the cylinders 34 and 36, the flow return to the reservoir being completed through the valve mechanism of the pump which includes passa e 96. space 95 beneath valve 104105, passages 97 and 98 to the return pipe 99 extending into the reservoir.

The electric motor 20 may be of any suitable type with its drive shaft 28 iournalled in a bearin provided by the housing 21. A duct 45 in the housing 21 leads from the shaft supporting bearing and terminates in the flat outer wall of the housing.

The flat outer wall of housin 21 is engaged by the cvlinder disc or plate 24, sometimes referred to as the pump barre clearly shown in Figs. 1, 2, 16 and 18. An annular recess 46, chamfered at its outer peripheral edge, is provided in the outer wall of housing 21 and contains a compressible ring gasket which is distorted bv the clamping of the plate 24 upon the housing 21, thereby sealing this co act area.

The cylinder plate or pump barrel 24. as shown in Figs. 16 and 18 has a substantially oval shaped opening 50 which forms the fluid displ cement chamber or cylinder of the pump. Two holes 51 in the plate 24, receive locating pins carried by the housing 21. Another hole 52 communicates with the duct 45 in the housing 21.

The fluid displacement member or piston, operative in the cylinder or pump barrel is in the form of a circular disc 53, operatively secured to the motor shaft 28 so as to be rotated thereby and fitting into the cylinder opening 59 as slidably to engage the two opposite wall portions of said opening more adjacent each other than those wall portions at thereto. As shown in Fig. 16, the piston disc 53 forms crescent shaped fluid displacement chambers at the two opposite and more remote end wall portions of the pump barrel 24. The piston disc 53 has a plurality of substantially radial slots 54, each of which slidably supports a vane 55, the outer ends of all vanes slidably engaging the inner wall of the cylinder opening 50 as the piston is rotated. As shown in Figs. 16 and 17, the slots are unequally spaced thus successive fluid pressure discharges occur at unequal intervals and therefore any undesirable efiects caused by rhythmic fluid pressure discharges as occur in pumps having symmetrically spaced vanes, is entirely eliminated. It will be noted also that the slots 54 are not absolutely radial of the piston disc, but on the contrary diametrically opposite slots are parallel to the diametral plane of the piston disc and set ahead of said plane, in the direction of piston rotation, a predetermined distance. This arrangement improves vane contact with the cylinder wall and in the use of ordinary square or slightly arcuated edged vanes, which are most economically produced, the possibility of end pressure to force the vanes inwardly of the slots is practically eliminated.

The port plate 25 is placed in contact with the cylinder plate or pump barrel 24. To prevent leaks between the contacting surfaces of plates 24 and 25, plate 25 has an annular groove 60 adjacent its outer, peripheral edge said groove 60 having its outer peripheral edge chamfered 'and receiving a compressible gasket ring, pressed into sealing engagement with both plates when they are clamped between housings 21 and 22. Figs. 13, 14 and 15 clearly illustrate the port plate 25. It has holes 61 coinciding with holes 51 in the cylinder plate to receive the locating pins in housing 21 and prop-' and .communicates with the fluid duct 52 in the cylinder.

disc.' Hole 62 terminates in a larger diameter recess 63 in the face of plate 25 contacting the pump. housing 22.

A central opening .64 in the port plate receives the end of the motor shaft 28. Around the opening 64 and coaxial therewith is a circular recess 65 formed in the face of plate 25 engaging the cylinder disc; Two diametrically opposed openings 66 and 67 in the bottom wall of recess 65 connect said recess with other ports or openings as will later be described. Four separate openings, equally spaced, are arranged in a circular row around the circular recess 65. The two diametrically opposite openings 70 and 71 are bisected by a diametral line at right angles to the diametral line bisecting the two openings 66 and 67 and bisecting the other two diametrically opposite holes 72 and 73 in the port plate. When the port plate 25 is placed in properly assembled position on the cylinder disc or plate'24, the web portions of the plate 25 between the ends of adjacent openings 70 to 73 inclusive segregate the four crescent shaped chambers formed in the cylinder space 50 by piston 53. Opening 70 in the port plate communicates with the right end of the upper crescent shaped displacement chamber as viewed in Fig. 16 which is a view looking into the pump cylinder from the motor end. Opening 72 communicates with the right end of the lower crescent shaped chamber, opening 71 with the left end of this lower chamber, and opening 73 with the left end of. the upper crescent shaped chamber.

Fig. shows the face of the pump housing 22 engaged by the port plate 25 when the mechanism is assembled. This face of the pump housing 22 has a circular groove 80'chamfered as the other similar grooves and which receives a compressible packing ring sealingly engaging both the housing and port plate when the two are clamped one on the other. An opening 81 coincides with the recess 63 in the port plate, said opening 81 communicating with a fluid passage 82 in'the pump hous ing, which passage communicates with the fluid return port with which pipe 32 communicates, see Fig. 2.

Asshown in Fig. 10, the face of the pump housing engaged by the port plate has an elongated, diametral recess 83 the ends of which align and connect with the two openings or ports 70 and 71 as shown in Fig. 12. Opening 64 of the port plate, into which the shaft 28 of the motor 20 extends with a clearing fit also opens into recess 83. The one end of recess 83 has an opening 84 in its bottom wall, said opening connecting the recess 83 with the passage'85,see Fig. 6. Pipe 86 fits into passage 85, said pipe extending downwardly from the pump housing 22 into the fluid tank or reservoir 27 attached to the base plate 26 of the pump housing. Pipe 86, termed the fluid inlet pipe hereinafter, is provided with a seat at its inner end, for the ball check valve 87 which, with the exception of a small by-passing orifice 88 in the seat, normally closes the inner end of the pipe. Inwardly extending ribs 89 in passage limit the movement of valve 87 from its seat during the time the pump is being operated by the electric motor 20 to draw fluid from the reservoir 27 and deliver it under pressure to work devices 34, 35 ,and 36 for actuating em. Thus valve 87 is operative to permit a substantially unrestricted fluid flow through passage 85 in one direction, that is, from the reservoir to the pump, however, if, under circumstances later to be described, reverse operation of the pump causes it to force fluid through passage 85 into the reservoir, the restriction to such a reverse flow, offered by the orifice 88, will resist the reverse operation of the pump and thus govern such reverse operation.

Two separate and diametrically opposed recesses 90 and 91 are provided in the face of the pump housing engaged by the port plate, these recesses aligning and communicating with the openings or ports 72 and 73 resplectively and also with the openings 66 and 67 respective y.

Recesses 90 and 91 differ in depth. A view of Fig. 11 shows recess 90 to be comparatively shallow while recess 91 is quite deep so as to communicate with discharge chamber 95 in housing 22, see also Figs. 19, 20 and 21. The shallow recess 90 with which port 72, in the port plate 25, communicates, in turn in communication with the circular recess 65 through opening 66. The opening 67 connects the circular recess 65 with the deeper recess 91 in housing 22. Thus thetwo recesses are interconnected, both discharging into the chamber 95.

Fig. 7 shows the chamber 95 in housing 22 as having two, superposed through passages 96 and 97 in communication therewith. The upper passage 96 is threaded at each end so as to receive pipes 30 and 31 connected to control devices for the work devices 34, 35 and 36. The lower passage 97 is also threaded at both ends for receiving pipes 32 and 33 which are the fluid flow return pipes from the various work devices. This lower passage 97 has a port 98 leading therefrom said port having a fluid return pipe 99 which extends into the fluid reservoir 27.

Chamber 95 being in communication with the discharge'port 91 of the pump, is provided with fluid flow control mechanism operative normally to shut off communication between the discharge port 91 and said chamber 95 and to maintain communication between the two through passages 96 and 97 so that fluid from the work devices may return to the reservoir 27 via port 98 and pipe 99. This fluid flow control mechanism in chamber 95 consists of a screw cap 100 threadedly received by and closing the outer end of chamber 95. The inner end of cap 100 is recessed to receive and seat the head of stud 101 extending centrally upwardly into chamber 95 for a portion of its length. A coil spring 102 surrounds the stud 101, the one end of the spring resting upon the head of the stud in plug 100. The other end of spring 102 abuts against the bottom edge of an inwardly extending annular flange 103 in the sleeve valve 104. The edge of flange 103 opposite the edge thereof engaged by the spring 102 is beveled to provide a conoidal seat for the ball check valve within said collar. The diameter of the opening defined by the flange 103 and normally closed by the ball valve 105 (see Fig. 19) is greater than the diameter of the stud or pin 101. Spring 102 urges the sleeve valve 104 upwardly in chamber 95 so that it normally abuts against the shoulder 106 in which position collar- 104 with its contained ball valve 105 closes communication between discharge port '91 and the passage 96, as shown in Fig. 19.

As shownin Fig. 20 the composite valve comprising the .sleeve valve and its contained ball valve will not open communication between the discharge port 91 and the through passage 96 until said sleeve valve portion 104 has entered the chamber portion between the upper and lower cross or through passages 96 and 97 at which time communication between said passages is cut 05. Communication between passages 96 and 97 is, however, reestablished when, upon continued downward movement of sleeve valve 104, .ball valve 105 engages the end of stud or pin 101, whereby movement of the ball is stopped and the sleeve valve consequently moves relatively to the ball to complete communication between said passages through the sleeve 104 as shownin Fig. 21.

As has been mentioned heretofore, work devices 34 and 36 may be untilized to raise and lower windows in an automobile. The Work device 35 may be used to raise and lower the top of a convertible type of automobile. Devices 34 and 36 are shown in Fig. 4 as hydraulic cylinders each having a piston which is actuated by fluid pressure in one direction and by a compressed spring in the other. Device 35, on the contrary, is fluid pressure actuated in both directions. Work devices 34 and 36 are each equipped with a control device 37 and 40 respectively. In one position these control devices shut off all fluid flow to and from their respective work devices. Each control device may be actuated into one position in which the electric motor is rendered active to drive the pump and the resultant fluid flow, under pressure may enter the cylinder to actuate the piston to raise the window. When the window is moved to the desirable position the control device is actuated to normal position again at which time the motor is rendered electrically inactive and the fluid flow to or from the respective cylinder is discontinued. To lower the window, the control device is moved from normal into another position by which the cylinder is in communication with the return flow passage 97 of the pump, permitting fluid from the cylinder to return to the reservoir 27 under the effect of the spring actuated piston in said cylinder.

The automobile top actuating device 35 has two control devices 38 and 39. Each is operative to render the motor active and to establish a fluid flowv from the pump to the cylinder 35 for actuating its piston in a respective direction in or out and at the same time to open the exhausting cylinder portion with the fluid flow return passage while the other cylinder portion receives fluid under pressure.

The motor 20 when active, rotates the piston or vane carrying disc 53 within the cylinder or pump barrel 24. Centrifugal force will initially hold the vanes 55 in contact with the cylinder wall 50 and thus fluid will be drawn from the reservoir 27 through pipe 86, past valve 87. through passage 85 into intake port or recess 83, thence through inlet ports 70 and 71 into the cylinder space between the respective vanes traversing the region of the inlet ports 70 and 71. As the fluid is displaced by the vanes into the region of the discharge ports 72 and 73 it passes through said ports. The fluid forced under pressure through port 72 flows through opening 66 into the circular recess or groove 65 thence through opening 67, joining the fluid flow from port 73 to enter the discharge port 91 which communicates with the passage 95 containing the fluid flow control mechanism.

Initially, as shown in Fig. 19, the fluid flow control mechanism and particularly the sleeve valve 104 and its contained ball check valve 105 shut off the discharge port 91 from the chamber 95, however, when the fluid pressure delivered by the pump reaches a predetermined value, sleeve 104 and ball 105 are moved as a unit into the position shown in Fig. 20 in which the discharge port 91 is in communication with the fluid passage 96. connected to the work device whose control device has been actuated to start the motor and to permit fluid to be directed to the respective cylinder comprising the work device. If for any reason, the pressure of the fluid delivered by the pump reaches an excessive value the sleeve valve 104 and its ball valve 105 will be moved further into chamber 95 until the ball rests upon the end of the stud or pin 101 at which time movement of the ball is arrested. The sleeve 104, however, may continue to be moved by this excessive fluid pressure, .its movement relatively to the ball openin up a passage through the sleeve as shown in Fig. 21. Now the pressure is relieved for some fluid from the pump may flow through sleeve 104 into the return passage 99 and thence to the reservoir 27.

When the particular window has been raised to the desired position, the control device, regulating the actuator or work device for this particular window, is operated to shut ofi fluid flow to the actuator and simultaneously render the electric motor electricallv innnerative. At this time the acting pipe or conduit. either 41 or 44, contains fluid under pressure wh ch. immediatelv upon storming of the pump bv electricallv disconnecting its driving motor, is directed through discharge passage 96, past the sleeve 104 now in the act of returning from its open position as in Figs. 20 or 21 into its shut-off position as in Fig. 19. This sudden surge of fluid under pressure is directed from the pump discharge port 91 through the pump, out of the inlet port of the pump through passage and pipe 86 to the reservoir. This sudden surge of fluid under pressure through the pump in the reverse direction and before the control mechanism comprising sleeve 104 and its ball valve 105 can return to full closing position results in the sudden reverse operation of the pump, acting now as a fluid motor driving the electric motor at comparatively high speed. Highly objectional belching noises result if such reverse flow of fluid into the reservoir due to back pressure is permitted. This invention avoids these objectional results by controlling the reverse flow. The ball check valve 87 in intake passage 85 restricts this return flow in accordance with the size of the by-pass orifice 88. This orifice 88 is calibrated to permit a reverse flow under these conditions which will rotate the pump reversely at a speed at which the undesirable noises are not produced. The return or reverse flow is so restricted by orifice 88 that spring 102 may normally return the sleeve valve 104 into its shut off position during which time the restricted fluid reverse flow past the closing sleeve is controlled to hold the speed of the reverse pump rotative within satisfactory limits to avoid the objec tionable belching noises mentioned.

Another feature of the present invention resides in the fact that all of the vane slots 54 have their inner ends in constant communication with the circular groove or recess 65 which is the connecting medium between the two discharge ports or recesses and 91, thus the vanes in the slots 54 constantly are urged outwardly against the wall 50 of the cylinder at existing fluid pressure developed by the pump.

The present device is also designed and constructed to avoid end thrust pressure upon the motor shaft. In the present device the end of the motor shaft upon which the pump piston disc 53 is secured extends through central opening 64 in the port plate and is therefore exposed to the fluid intake recess 83 in the pump housing.

The present invention provides a unitary power driven pump and fluid supply of simple structure and design, capable of being commercially produced at a minimum cost of time and material said unitary device functioning smoothly and noiselessly.

While the embodiment of the present invention as herein disclosed, constitutes a preferred form, it is to be understood that other f rms might be adopted.

What is claimed is as follows:

1. In a power driven pump operative to draw fluid from a reservoir and deliver it under pressure to an actuator, the combination with fluid flow passages connecting the pump with the fluid reservoir and with the actuator; of fluid flow control means in the fluid passage connecting the pump and reservoir, said flow control means having one automatically attained position for substantially unrestricted fluid flow from the reservoir to the pump while said pump is being driven and having a second automatically attained position providing for a predetermined continuously open restriction to the flow of fluid under pressure from the actuator through said pump into the reservoir when the pump ceases operation.

2. A unitary fluid displacement device comprising in combination, a fluid pump including a fluid displacement mechanism, a motor connected to the pump for driving it; a fluid reservoir attached to the pump; a conduit connecting the reservoir with the fluid displacement mechanism of the pump; a fluid passage for connecting said mechanism with an actuator operative by the fluid pressure delivered by the pump; and a fluid flow control device in said conduit, said device being operative in one automatically attained position to provide a substantially unrestricted fluid flow from the reservoir to the pump as the latter is being driven by the motor and being operative in a second automatically attained position to establish a fixed orifice to predeterminately restrict reverse fluid flow under pressure from the actuator through said conduit from said pump upon cessation of pump operation by the motor.

In a power driven pump operative to draw fluid from a reservoir and deliver it, under pressure, to a fluid operated actuator, the combination with a fluid flow control mechanism interposed between the pump and actuator; of a fluid flow control device interposed between the pump and reservoir, said device including valve means having an open position in which it is operative to provide a substantially unrestricted fluid flow from the reservoir to the pump and including an open by-pass passage by-passing said valve means in closed position of the said valve means to restrict predeterminately any flow of fluid in the opposite direction.

4. In a power driven pump operative to draw fluid from a reservoir and deliver it under pressure to a pressure actuated work device, the combination with a valve mechanism yieldably urged, normally to shut off communication between the pump and the pressure actuated work device in one position thereof and having another position at which the fluid delivery pressure reaches a predetermined value for flow of fluid under pressure to the work device; and valve means interposed between the pump and reservoir and having a restrictive by-pass passage around the valve means, said valve means having an automatically attained open position to permit a substantially unrestricted flow of fluid from the reservoir to the pump while the pump is power actuated and an automatically attained closed position to eflect restriction to the return flow of fluid under pressure from the work device, through the pump, to the reservoir through the by-pass when the pump ceases being power actuated and while the valve mechanism is moving from the said other position to the said one position.

5. In a power driven pump operative to draw fluid from a reservoir and deliver it under pressure to a pressure actuated work device, the combination with a spring loaded valve having a first position normally shutting ofl communication between the pump and work device but operative to a second position at a predetermined fluid pressure developed by the pump to connect the pump with the work device for actuating the latter; and an orificed valve interposed between the reservoir and pump, said orificed valve having a first automatically attained open position permitting a substantially unrestricted fluid flow from the reservoir to the pump While the latter is operating under power to actuate the work device, and having a second automatically attained closed position in which the orifice of the orificed valve restricts the return flow of fluid under pressure from said work device through the pump to the reservoir when the pump ceases operation and while the spring loaded valve moves from its second to its first position.

6. In a power driven pump operative to draw fluid from a reservoir and deliver it under pressure to a fluid pressure actuated work device, said pump having fluid intake and discharge ports, the combination with a spring loaded valve in the pump, normally shutting ofl the discharge port of the pump to prevent communication between the discharge port of the pump and the work device, but operative at a predetermined fluid pressure developed by the pump to open the discharge port, to connect said discharge port with the work device for actuating the latter; of a check valve interposed between the reservoir and intake port of the pump, said check valve permitting fluid to flow from the reservoir to the pump only as the latter is being power operated; and an open fluid by-pass passage around said check valve, to permit a predeterminately restricted return flow of fluid from the work device, through the pump into the reservoir when the pump ceases to be power actuated and while the spring loaded valve returns to its normal position.

7. In a unitary fluid transfer device comprising an electric motor, a pump connected to and driven by said motor and a fluid containing reservoir, the combination with a check valve between-the fluid reservoir and inlet of the pump, said check valve having one position to permit a substantially free flow of fluid from the reservoir to the pump and a second position to close off the said flow; and an orifice in the seat of the check valve, operative to by-pass the check valve when the pump ceases to be driven by the motor, to restrict reverse flow of fluid under pressure from the fluid pressure actuated device, through the pump and into the reservoir whereby to retard temporary reverse operation of the pump and motor.

References Cited in the file of this patent UNITED STATES PATENTS 1,861,821 Schaum June 7, 1932 1,974,775 Gorsuch Sept. 25, 1934 1,989,900 Vickers Feb. 5, 1935 2,020,618 Persons Nov. 12, 1935 2,389,164 Payne Nov. 20, 1945 2,566,745 Parsons Sept. 4, 1951 2,586,682 McLeod Feb. 19, 1952 

